Gluconeogenesis Flashcards
Function of gluconeogenesis
Another mechanism of maintaining blood glucose homeostasis in addition to glycogen degradation
When
FASTED state
- Low I/G as in overnight fast
- Prolonged exercise
- High protein, low-carb diet
- Stress
Where
Primary site = liver
Also in kidney to lesser extent
Liver cannot carry out _________
Glycolysis and gluconeogenesis at same time at same rate.
Glycolysis ___________ in the fasted state in ________
Decreases
the liver
Major non-carbohydrate precursors used as substrates (4)
- Lactate
- Amino acids
- Glycerol
- TCA cycle intermediates
All TCA cycle intermediates except ______ can be used as substrates in gluconeogenesis
Acetyl CoA
Cori Cycle =
Lactate recycling
2 functions of lactate dehydrogenase
- Maintain glycolysis during fed state (especially under anaerobic conditions) by producing NAD+
- Provide lactate for gluconeogenesis in fed state
Chemistry of forming pyruvate
Enzyme: lactate dehydrogenase
Oxidize -OH group on lactate to C=O —>form pyruvate
NAD+ picks up e- to NADH
Lactic acid produced as a consequence of anaerobic glycolysis in muscle enters blood stream —>
Taken up by liver —> converted to pyruvate by lactate dehydrogenase —> pyruvate serves as carbon skeletons for gluconeogenesis
Where does lactate come from? (2)
- RBC
2. Exercising muscle
Where do amino acids come from?
Muscle protein degradation in skeletal muscle
Primary amino acid used to make pyruvate?
How?
Alanine
Alanine aminotransferase converts to pyruvate by removing amino group
Where does glycerol come from?
Triglyceride breakdown in adipose tissue
Use of glycerol as a substrate
Glycerol + ATP —> glycerol phosphate + ADP + Pi (enzyme: glycerol kinase)
* glycerol is from stored TG here
Glycerol phosphate + NAD+ —> dihydroxyacetone phosphate + NADH
Dihydroxyacetone phosphate
A glycolytic intermediate
Generated by using glycerol from breakdown of stored triglycerides
_________ carbons cannot be used as substrates for gluconeogenesis
Why?
Fatty acid
Most FA carbons only yield acetyl CoA —> acetyl CoA through TCA cycle cannot provide for a net synthesis of sugars because 2 carbons are lost as CO2
Also pyruvate dehydrogenase reaction is irreversible
4 main molecules in order
Pyruvate —> OAA —> phosphoenolpyruvate —> glucose
Any ATP is coming from?
Oxidation of fatty acids
Glycolysis is shut down because?
Pyruvate kinase is phosphorylated and inhibited in fasted state
Retained steps
2, 4-9
Replaced steps
1, 3, 10
Step 1
Pyruvate + CO2 + ATP + H2O —> OAA + ADP + Pi + 2 H+
Enzyme: pyruvate carboxylase
- Add CO2 to pyruvate
- Occurs in mitochondria
Pyruvate carboxylase requires
Biotin as a coenzyme for carrier of CO2
Source of CO2 is bicarbonate
What provides energy for gluconeogenesis?
Beta-oxidation of fatty acids
- must happen at same time
Allosteric activators of pyruvate carboxylase
- Acetyl-CoA : comes from FA oxidation
2. ATP
Step 2
[First, OAA is transported into cytoplasm as malate and OAA is reformed.]
OAA +GTP —> Phosphoenolpyruvate + GDP + CO2
Enzyme: phosphoenolpyruvate carboxykinase (PEPCK)
Why is GTP needed in PEPCK reaction?
Forming phosphoenolpyruvate which is a high energy molecule
Energy for forming PEP is provided by:
- Hydrolyzing GTP
2. Decarboxylation is favorable (remove CO2 that pyruvate carboxylase added)
Step 3
PEP + H2O 2-phosphoglycerate
Step 4
2-phosphoglycerate 3-phosphoglycerate
Step 5
3-phosphoglycerate + ATP 1,3-BPG + ADP
Step 6
1,3-BPG + NADH Glyceraldehyde-3-phosphate + NAD+ + Pi
***Also dihydroxyacetone phosphate enters here by isomerizing to form glyceraldehyde 3-phosphate
Step 7
Glycerol-3-phosphate fructose-1,6-bisphosphate
Step 8
CONTROL POINT
Fructose-1,6-bisphosphate + H2O —> fructose-6-phosphate + Pi
Enzyme: Fructose-1,6-bisphosphatase
- Thermodynamically favorable
Fructose-1,6-bisphosphate is allosterically inhibited by:
- Fructose-2,6- bisphosphate
2. AMP
Step 9
Fructose-6-phosphate glucose-6-phosphate
Step 10
Glucose-6–phosphate + H2O —> glucose + Pi
Enzyme: glucose-6-phosphatase
Glucose-6-phosphatase
- location
- mechanism
- Found in ER membrane
- Catalytic domain faces ER lumen
Glucose-6-phosphate gets into ER lumen of liver and kidney cells —> P comes off in hydrolysis reaction —> both P and glucose exit lumen —> glucose into blood
4 major enzymes in gluconeogenesis
- Pyruvate carboxylase
- PEPCK
- Fructose-1,6-bisphosphatase**
- Glucose-6-phosphatase
3 major enzymes in glycolysis
- Pyruvate kinase
- PFK-1**
- Glucokinase / hexokinase
PFK-1 allosteric regulation
Allosterically activated by F-2,6-BP and AMP
Allosterically inhibited by ATP
Fructose-1,6-bisphosphatase allosteric regulation
Allosterically inhibited by F-2,6-bisphosphatase and AMP
Pyruvate allosteric regulation
Activated by fructose-1,6-bisphosphate
Inhibited by ATP
Phosphoenolpyruvate carboxykinase (PEPCK) allosteric regulation
Inhibited by ADP
Pyruvate carboxylase allosteric regulation
Activated by acetyl CoA
Inhibited by ADP
Glucose-6-phosphatase regulation
Under substrate level control
If G-6-P builds up, increase activity of enzyme
Fructose-2,6-bisphosphatase is made by ______ in _________
PFK-2 (kinase domain) in fed state
In fasted state, PFK-2 is ___________ and ________ active
Result:
Phosphorylated and less active
Result: Allows FBPase-2 to become more active
In fasted state __________ degrades __________
FBPase-2 degrades fructose-2,6-bisphosphate
In fed state, insulin signaling activates _________ to _________ PFK-2 to make ____________
In fed state, insulin signaling activates phosphoprotein phosphatase to remove phosphate from PFK-2 to make it active —> make fructose-2,6-bisphosphate
FBPase-2 is _________ by fructose-2,6-bisphosphate
Inhibited
Need _____ high energy phosphate to make 1, 6-C glucose
Where do they come from?
6
2 ATP in Pyruvate carboxylase reaction
2 GTP in PEPCK reaction
2 ATP as 2 moles of 3-phosphoglycerate —> 2 mol 1,3-BPG
ATP in fasted state
NOT limited
Why? —> Fatty acids give more ATP than glucose
Glycolysis = ________ energy
Gluconeogenesis = _________ energy
Glycolysis releases energy (ATP and NADH)
Gluconeogenesis consumes ATP and NADH
Glycolysis and gluconeogenesis are ________ regulated.
Reciprocally
